1. Field of the Invention
The present invention relates to a semiconductor switching device such as a thyristor and a method of controlling a carrier life time in a semiconductor switching device.
2. Description of the Background Art
It is known in the art to control a carrier life time in a semiconductor switching device by irradiating the semiconductor switching device with a radiation, such as an electron beam, a proton beam, alha rays and gramma rays, to cause radiation defects, or lattice defects in wide sense. An example, in which a buried gate type static induction thyristor (hereinafter referred to as an SI thyristor) is irradiated with a radiation, will be described.
FIG. 1 is a cross-sectional view schematically showing the main portion of a structure of a conventional SI thyristor which has radiation defects caused by irradiation with a radiation. FIG. 2 is a view showing a process of irradiating an SI thyristor with a radiation.
Referring to FIG. 1, the SI thyristor comprises an N type semiconductor substrate 1, an anode region 2 which is formed on the back surface of the semiconductor substrate 1 by diffusing P type impurities, a cathode region 3 which is formed on the top surface of a jut of the semiconductor substrate 1 by diffusing N type impurities, and gate regions 4 which are formed on step portions of the semiconductor substrate 1 by diffusing P type impurities. A main current 9 flows through a channel region 5, which is surrounded by the gate regions 4.
This SI thyristor is irradiated with a radiation 10 such as an electron beam to cause radiation defects 7 in the semiconductor substrate 1, as shown in FIG. 2. The radiation defects 7 are uniformly distributed all over the irradiated portion of the semiconductor substrate 1, since the radiation defects 7 are caused by the interaction of the radiation 10 and atoms forming the semiconductor substrate 1. In FIG. 1 and FIG. 2, however, the radiation defects 7 are shown by only some cross marks for the convenience of illustration.
The operation of the SI thyristor, in the condition that a carrier life time is uniformly decreased all over the semiconductor substrate 1 due to the radiation defects 7, will now be roughly described. In this context "carrier life time" is directed to how long carriers can live on the average. This is a matter of probability as to whether or not carriers can be left alive to reach the main electrod without being trapped by defect. In an ON state, the main current 9 flows from the anode region 2 to the cathode region 3 through the channel region 5 against the interruption of the radiation defects 7, while many carriers 8 are trapped by the radiation defects 7. At the moment of a turn OFF of the SI thyristor, a number of carriers remain in the semiconductor substrate 1. If the life time of the carriers is long, the substantial turn OFF time of the SI thyristor becomes long. However, the SI thyristor shown in FIG. 1 has a number of radiation defects 7 which serve as a life time killer to the carriers, and hence the turn OFF time of the SI thyristor is decreased in the form of the decrease of a tail current. Thus, the switching loss of the SI thyristor in a turn OFF transistion is greatly decreased by irradiating the SI thyristor with the radiation 10 to control a carrier life time.
However, such uniform irradiation of the semiconductor substrate 1 with the radiation 10 causes a number of radiation defects 7 in the channel region 5 through which the main current 9 flows, so that the forward voltage of the SI thyristor in an ON state is disadvantageously increased, mainly due to the decrease of a carrier life time in the channel region 5. The increase of the forward voltage results in the increase of a power loss in an ON state. Especially in a high breakdown voltage device such as a device having a higher breakdown voltage than 1500 V, such increase of the forward voltage causes unacceptable ON state power loss and exothermic device destruction, since such a device generally has a substrate having a high specific resistance.
Further, at the initial stage of a turn ON transition, carriers must progress against the radiation defects 7, in other words, the main current 9 must flow in the substrate 1 having a specific resistance increased by the radiation defects 7, and hence the turn ON time of the SI thyristor is increased.
If the SI thyristor is uniformly irradiated with a lot of radiation 10 to perform a strong life time control, a leak current in an OFF state is increased due to the occurrence of defects in a passivation film (not shown) and/or the deterioration of a surface condition of the substrate 1, and a main breakdown voltage is decreased due to the increase of a leak current at the exposed edges of P-N junctions defined by the substrate 1 and the gate regions 4.
In a MOS type transistor, such as MOSFET, other than a bipolar type high breakdown voltage switching device such as an Insulated Gate Bipolar Transistor (hereinafter referred to as an IGBT), a gate portion includes an insulator such as a silicon oxide film. Therefore, if the MOS type transistor is irradiated with a radiation to control a carrier life time, various surface states, which are induced in the gate portion, make controlling through a gate difficult, in addition to the problems as hereinbefore described.
Thus, the life time control of a semiconductor switching device by the irradiation of a radiation such as an electron beam has the advantage of decrease of a turn OFF time and the disadvantages of increase of a turn ON time as well as a forward voltage, and in some cases decrease of a breakdown voltage. The advantage and disadvantages are in a trade-off relationship, so it is difficult to appropriately accommodate the trade-off relationship for particular use of the semiconductor switching device.
A method of partly recovering deteriorated characteristics is conventionally widely used, wherein a semiconductor switching device irradiated with a radiation is subjected to an annealling process to recover a crystal structure. Through the annealling process, the deteriorated characteristics are uniformly recovered, since a whole semiconductor switching device is only put into an uniformly high temperature condition in the anneal processing. Therefore, this annealling process is equivalent to work for selecting optimum quantity of irradiation of a radiation to implement a device having desired characteristics, and hence the problems as hereinbefore described can not be solved in essence through the annealling process.